Recent regulations promulgated by the Federal Communications Commission (FCC) require wireless telephone service providers within the United States to implement Emergency 911 location service for identifying the location of a mobile user making a 911 call. In providing such service, a location measurement unit (LMU) antenna is used, wherein the LMU antenna in the communications system must be isolated from co-located transmitting antennas so that signals from neighboring cell sites are not drowned out. Although physically separating the LMU antenna from co-located antennas on an antenna tower may provide some isolation, the limited space on typical antenna tower platforms prevents physically separating such antennas by distances great enough to provide necessary isolation.
Isolation of an auxiliary antenna, such as an LMU antenna, from a main antenna, such as a base station antenna, mounted within a common antenna assembly is non-trivial, particularly when the transmitting and/or receiving frequency range of the auxiliary antenna at least partially overlaps the transmitting and/or receiving frequency range of the main antenna.
The present invention is accordingly directed to an antenna system for isolating an auxiliary antenna, such as an LMU antenna, from a main antenna, such as a base station antenna, mounted within a common antenna assembly, and also from other co-located antennas mounted to an antenna tower.
The present invention comprises one or more of the following features or combinations thereof. A main antenna, such as a base station antenna, and an auxiliary antenna, such as an LMU antenna, are mounted within a common antenna assembly. The main antenna may be configured to transmit or receive signals in a first range of radio frequencies, and to develop a main beam that is substantially wider in azimuth than in elevation. The main beam may define a beam elevation configured to communicate with mobile terminals. The auxiliary antenna may be configured to transmit or receive signals in a second frequency range at least partially overlapping the first frequency range, and to develop an auxiliary beam at least partially overlapping the main beam. The auxiliary antenna may be configured to communicate with co-located or remote base station antennas. The auxiliary beam may be substantially wider in azimuth than the main beam, and/or may be omni-directional. The auxiliary antenna may be positioned elevationally above or below the main antenna.
The main and auxiliary antennas may define a space therebetween sized to decouple the main and auxiliary antennas and minimize interference therebetween. The space may include a radio frequency energy absorbing member and/or a radio frequency energy scattering member operable to decouple the antennas to suppress interference between the main and auxiliary beams. The radio frequency energy absorbing member may be formed of a material configured to absorb energy in the second frequency range. The radio frequency energy scattering member may be a radio frequency choke structure which may comprise a body defining at least one slot between a pair of electrically conductive plates each defining a channel therethrough, each of said plates defining a length of about one-quarter of the wavelength of said second frequency range between an outer periphery thereof and an outer periphery of said channel.
The auxiliary antenna may comprise one or more radiator elements that may be designed so as to minimize transfer of energy to the main antenna, for example, by suppressing the signals radiated by the auxiliary antenna in the direction of the main beam of the main antenna.
The auxiliary antenna may include one or more energy absorbing members positioned about the one or more radiator elements to absorb energy in the second frequency range transmitted or received by the auxiliary antenna to thereby isolate the auxiliary antenna from other co-located antennas.
The main antenna may be positioned adjacent to a first ground plane and the auxiliary antenna may be positioned adjacent to a second ground plate isolated from the first ground plate. The main antenna may or may not be mounted to the first ground plate, and the auxiliary antenna may or may not be mounted to the second ground plate.
An electrically non-conductive support structure may be provided to interconnect the main and auxiliary antennas by uniting the first and second ground plates and/or the main and auxiliary antennas. The non-conductive support structure may comprise an electrically non-conductive radome surrounding the main and auxiliary antennas and/or at least one electrically non-conductive elongated member interconnecting the first and second ground plates and/or the main and auxiliary antennas.
Such an antenna system may comprise part of a multi-antenna installation having an antenna tower including a number of antenna mounting platforms each having one or more signal receiving and/or signal transmitting antennas mounted thereto. Such an antenna system may be mounted to any one of the number of antenna mounting platforms.
These and other features of the present invention will become more apparent from the following description of the illustrative embodiments.